According to the current state of art the configuration commonly used for the placement of the electrodes in a transverse discharge gas pumped laser is illustrated in FIG. 1 showing two opposite electrodes placed side by side to conductors for the back current. The operation of a transverse discharge laser of this kind can be easily illustrated with reference to the diagram of FIG. 2. A suitable differential voltage is applied across the two opposite electrodes E1, E2 by means of a condenser bank charged at the voltage V.degree.. The gas made previously conductive by preionization with UV-fotons or X-rays is crossed by a high electric pulse current transferring to the gas the necessary energy to bring atoms or molecules at the excited level from which they should decay to provide the laser effect.
This geometry known as TEA (Transversely Excited Atmospheric-pressure) is used either when very short excitation time of the gas or short laser pulses (CO.sub.2) are desired or because the discharge in the used gas is not stable so that the discharge must finish before the instability condition is established (excimer laser). During the discharge the system can be used as a RLC circuit, where C essentially is the condenser bank charged at the voltage V.degree., L is the inductance of the connection between condenser bank and laser head, R is the resistance of the discharge. It is then evident that the transverse discharge arrangement, with respect for example to the longitudinal discharge arrangement, allows both L and R to be reduced under the same active gas volume and energy stored in the condenser bank. Therefore, energy is more rapidly transferred to the gas.
The transverse discharge arrangement has, however, along with the previously described advantage some negative aspects summarized herebelow:
1) The two electrodes have to be sidewise integrated with conducting studs covered by an insulating layer which should provide the correct path of the current. These studs should be near the electrodes to minimize, but not to a great extent, the inductance in order to avoid electrical discharge problems which can be only partially avoided by the insulating covering. Furthermore, the construction of the studs should be highly transparent to the gas flow (large void-solid content ratio, i.e. studs spaced a lot, net with spaced meshes), which is in contrast with the necessity of the discharge circuit to have a low impedance (i.e. requesting a construction with opposite features). Furthermore, the studs should be aerodinamically profiled (i.e. a complicated machining) to minimize the turbulence in the active volume, thus impairing the optical quality of the laser beam. Again, when the insulating material covering the studs is hit by the hot and reactive gases after the discharge, chemical reaction changing the useful mean-life of the gas can easily occur (thus increasing the operation cost due to the increase of spares).
2) The pumping technique of an excimer laser by transverse discharge requires the use of switches able to withstand high peak currents (up to 100 KA) with high current derivatives (for example dI/dt greater than 10.sup.12 A/s) to have a high efficiency. The employed solutions always are a compromise. In fact:
the use of thyratron, as it is widely made, reduces the efficiency due to the limited derivative dI/dt; again, the use of the characteristics at their limits reduces the mean-life;
the use of a spark gap allows the requested performance in terms of dI/dt and Imax to be achieved, but the mean-life of such means is always limited (max. 10.sup.3 pulses) and this is a great limit for sources designed to operate at high repetition frequency (for example, in a source operating at 1 kHz the mean-life of the spark gap would be about 30 working hours).
switches with saturable magnets can be used. In such a case the requested values of Imax and dI/dt can be obtained as well as a mean-life compatible with a prolonged use, but such systems are cumbersome, expensive and above all less flexible as they are designed for a determined working point (in terms of energy transferred by each pulse, operation voltage, a.s.o.) which cannot be easily changed.
3) It should be added that a device used by Long for the first time and successively by many others allows the performance of the excimer lasers to be expanded. Such a device is known as "prepulse" circuit. The disclosure of such a device is made evident by the normal operation mode of a transverse discharge laser. In such lasers the discharge process develops through two separate phases. In fact, the preionization sources currently used produce an electron density of 10.sup.7 to 10.sup.11 e/cm.sup.3, which is far away from the requested density of the discharge (10.sup.14 -10.sup.15 e/cm.sup.3).
In the initial phase of the discharge (&lt;20 ns) a high electrical field is requested in order to increase the density of the electrons, and in the subsequent phase a much lower electrical field holding the discharge is requested. This double function is normally performed by only one circuit using in practice the impedance variation of the discharge and an auxiliary condenser of low capacitance (peaking condenser) to produce an electrical field varying by about a factor 2 from the inital phase to the following. The ideal value is, however, greater than 3. Under ideal conditions, in the Long's work two circuits operating in succession have been used (see FIG. 3): at the beginning switch I.sub.0 connecting condenser C.sub.0 charged at high voltage V.sub.0 to small capacities is closed, thus providing the high electrical field of multiplication, and immediately afterwards switch I.sub.1 connected to condenser C.sub.1 of great value and charged at a much lower voltage is closed, thus providing the most of gas excitation energy. By this contrivance, i.e. by separately optimizing the electrical fields of the multiplication phase and discharge phase, two important results have been achieved: the efficiency of an excimer XeCl-laser has been brought from 2% to 4.2% (Long et al.), and the duration of the laser pulse has been brought from about 150 ns to over 500 ns. The use of such contrivance always requires the use of an auxiliary switch (spark gap, saturable inductance, ausiliary electrode) which isolates the electrode of the condenser bank when the multiplication pulse is applied.
4) In order to complete the outline of the present state of art it should be appreciated that the socalled MOPA (Master Oscillator Power Amplifier) construction has been resorted to, in which two laser chambers are operated at the same time in order to satisfy the request of beams of very high quality (narrow spectrum, low divergency). According to such construction the beam with the requested features is formed in a master oscillator (by means of rasters, etalon, unstable cavities, a.s.o.). Such beam is then amplified by a power amplifier. Such circuit requires for its operation a low power unit (not necessarily at high efficiency) synchronized with accuracy with a power unit having high efficiency. Of course, except for the advantage of the double discharge region, the system suffers from all of the previously described problems.